EP0417680A1 - Method for the production of 1,1,1,2-tetrafluoroethane - Google Patents

Abstract

The invention relates to a method for the production of 1,1,1,2-tetrafluoroethane from 1,1,1-trifluoro-2-chloroethane and hydrogen fluoride in the gas phase. This method makes use of a catalyst which contains chromium and magnesium and which can be obtained by precipitating chromium(III) hydroxide by reacting 1 mole of a water-soluble chromium(III) salt with at least 1.5 moles of magnesium hydroxide or magnesium oxide in the presence of water, converting the reaction mixture into a paste which contains chromium hydroxide and a magnesium salt, and then drying the paste and treating the product with hydrogen fluoride at temperatures of 20 to 500 DEG C.

Description

The invention relates to a process for the preparation of 1,1,1,2-tetrafluoroethane by reacting 1,1,1-trifluoro-2-chloroethane with hydrogen fluoride in the gas phase.

1,1,1,2-tetrafluoroethane (also referred to below as tetrafluoroethane) can be used as a chlorine-free substitute for difluorodichloromethane (R 12) in refrigeration and air conditioning technology.

It is already known that tetrafluoroethane can be obtained in the reaction of 1,1,1-trifluoro-2-chloroethane (hereinafter also referred to as trifluorochloroethane) with hydrogen fluoride over suitable catalysts in the gas phase as well as in the liquid phase. Solids have mainly been described as catalysts for the gas phase reaction. which either consist entirely of chromium (III) compounds or contain a chromium (III) salt on a carrier material such as aluminum oxide. DE-OS 2 806 865 (= US Pat. No. 4,129,603) describes the use of a chromium oxide catalyst which is preferably obtained by treating a chromium hydroxide paste with steam and then converted to a chromium oxyfluoride using hydrogen fluoride. DE-PS 2 932 934 describes the use of various chromium (III) compounds as catalysts, e.g. Chromium (III) fluoride or other salts that are converted to chromium oxyfluoride or chromium (III) fluoride by hydrogen fluoride treatment.

The processes mentioned still have various disadvantages, which are particularly unfavorable for the industrial production of tetrafluoroethane. In addition to tetrafluoroethane, other compounds are formed, among others 1,1-difluoro-2-chloroethene, which is difficult to separate from the desired product by distillation.

According to the process of DE-OS 2 806 865, either a second reactor is required in order to react the 1,1-difluoro-2-chloroethene contained in the crude product again at lower temperatures, or it has to be separated by wet-chemical means by a permanganate solution, which requires considerable additional effort in both cases. The selectivities that are achieved with the chromium oxyfluoride catalyst before the separation of 1,1-difluoro-2-chloroethene are 91-95%.

The catalysts described in DE-PS 2 932 934 briefly achieve a higher selectivity for tetrafluoroethane (up to 98%) at temperatures of around 400 ° C., the residual content of 1,1-difluoro-2-chloroethene not being stated. However, the chromium (III) catalysts used lose noticeable activity within 44 hours, which can be compensated for by the continuous metering of a certain amount of oxygen. However, the additional metering of oxygen into the reactor, in addition to the increased technical outlay, also brings with it other problems when working up the product. Oxygen complicates the compression or condensation of the product and, when dissolved, is very disadvantageous when tetrafluoroethane is used in refrigeration and air conditioning technology. If oxygen is overdosed into the reactor, the selectivity for tetrafluoroethane also decreases again.

The one by Marangoni et. al. (Chim. Ind. 64 , 135 (1982)) described catalyst from precipitated chromium hydroxide only achieves a tetrafluoroethane selectivity of 79% at 350 ° C. for 60 hours.

It has now been found that the catalyst described in EP-PS 130 532 (= US Pat. No. 4,547,483}) consists of Magnesium fluoride and a fluorine-containing chromium (III) compound, which enables the conversion of trifluorochloroethane into tetrafluoroethane surprisingly selectively and with longer-lasting activity. In the as yet unpublished German patent application P 39 23 256.5, this catalyst is used to produce the starting material 1,1,1-trifluoro-2-chloroethane.

The invention relates to a process for the preparation of 1,1,1,2-tetrafluoroethane from 1,1,1-trifluoro-2-chloroethane and hydrogen fluoride and the gas phase, characterized in that a chromium and magnesium-containing catalyst is used which is obtainable by precipitating chromium (III) hydroxide by reacting 1 mol of a water-soluble chromium (III) salt with at least 1.5 mol of magnesium hydroxide or magnesium oxide in the presence of water, converting the reaction mixture into a dough, the chromium hydroxide and Contains a magnesium salt, and then dries the dough and treated with hydrogen fluoride at temperatures of 20 to 500 ° C.

The catalyst for the gas phase reaction according to the invention is particularly effective under increased pressure, since it greatly reduces the content of olefinic by-products in the product mixture, in particular 1,1-difluoro-2-chloroethane. In this way, the olefin content in the crude product can be reduced to values below 100 ppm without using a second reactor with a lower temperature or chemical washing, as described in DE-OS 2 806 865. During the reaction, a pressure of 1-26 bar, preferably 5-15 bar, is set in the reactor by means of a control valve.

In the gas phase reaction according to the invention, the catalyst shows over a longer period of time than the catalyst described in DE-PS 2 932 934 no loss of activity, so that it can do without the additional metering of oxygen and thus the technical disadvantages mentioned are eliminated.

The process according to the invention can be carried out, for example, in such a way that the starting materials trifluorochloroethane and hydrogen fluoride are fed continuously into an evaporator made of stainless steel or nickel. The temperature of the evaporator is not critical, but must be sufficient to completely convert both components into the gas phase at the selected pressure. The gaseous starting materials, possibly via a preheating section and a gas mixer, enter the reactor which contains a bed of the catalyst described in EP-OS 130 532. The reactor is also made of stainless steel or nickel and can be used in various technical versions, e.g. as a shaft, tube or annular gap reactor.

The temperature of the catalyst bed is kept at 250-450 ° C., preferably at 320-380 ° C., by heating the reactor.

In order to achieve the highest possible conversion of trifluorochloroethane and a very high selectivity for tetrafluoroethane at the stated reaction temperatures, the hydrogen fluoride is preferably used in excess. The molar ratio of hydrogen fluoride / trifluorochloroethane is generally. at least 1: 1; the molar ratio is only limited by economic considerations. It is preferably 2: 1 to 10: 1, in particular 4: 1 to 8: 1.

Unreacted trifluorochloroethane can be returned to the reactor.

The process according to the invention is explained in more detail by the following examples.

Test report A (catalyst production according to EP-PS 130 532)

200 g (Cr (NO₃) ₃ x 9 H₂O were dissolved in 1 liter of water. This solution was added to a mixture of 500 g magnesium oxide and 240 g graphite and the resulting paste-like mass was kneaded intimately.

The pasty reaction product was then granulated into cubes (0.5 cm edge length) and dried at 100 ° C. for 16 hours.

1 l (bulk volume) of the dried catalyst bodies (= 600 g) were treated with 15 mol of hydrogen fluoride in a tube made of nickel or VA steel with a clear width of 5 cm and a length of 100 cm at 200 ° C. The duration of the hydrogen fluoride treatment was approximately 6 hours. The hydrogen fluoride was diluted with N₂. The fluorination catalyst obtained had a chromium content of 2.3% by weight.

B (catalyst production according to EP-PS 130 532)

200 g of Cr (NO₃) ₃ x 9 H₂O were dissolved in 278 ml of water. This solution was added to a mixture of 138 g magnesium oxide and 136 g graphite. Further processing and hydrogen fluoride treatment were carried out according to test report A. The finished fluorination catalyst contained 4.3% by weight of chromium.

example 1

120 g of trifluorochloroethane and 120 g of hydrogen fluoride (molar ratio 1: 5.9) per hour in the gaseous state were produced over 1 l of the according to test report A. Catalyst passed in a tubular reactor, which was kept at a temperature of 360 ° C by an electrical heating winding.

The tubular reactor was the same one that was already used for hydrogen fluoride treatment in the manufacture of the catalyst.

The gaseous reaction products leaving the reactor were fed to a washing tank filled with water or potassium hydroxide solution, in which the hydrogen chloride formed and excess hydrogen fluoride were collected.

The gaseous, water-insoluble reaction products were analyzed by gas chromatography.

After 6 hours at normal pressure, the conversion was 27.4%, based on the trifluorochloroethane used. The selectivity for tetrafluoroethane was 96.4%, based on the trifluorochloroethane converted.

The reaction product contained 26.4 wt .-% CF₃-CH₂F, 0.23 wt .-% CF₂ = CHCl and 0.77 wt .-% other components besides unreacted CF₃-CH₂Cl.

Example 2

The catalyst prepared according to test report A was used in the same test arrangement as in example 1 for the reaction of trifluorochloroethane with hydrogen fluoride. 120 g per hour of trifluorochloroethane and 120 g per hour of hydrogen fluoride were passed over the catalyst in gaseous form at 360 ° C. and 10 bar pressure.

After 20 hours, the conversion was 26.6%, based on the trifluorochloroethane used. The selectivity for Tetrafluoroethane was 97.5% based on the trifluorochloroethane converted.

The reaction product contained 25.9 wt .-% CF₃-CH₂F, 0.009 wt .-% CF₂ = CHCl and 0.69 wt .-% other components in addition to unreacted CF₃-CH₂Cl.

Example 3

The catalyst prepared according to test report A was used in the same test arrangement as in example 1 for the reaction of trifluorochloroethane with hydrogen fluoride. 120 g per hour of trifluorochloroethane and 160 g per hour of hydrogen fluoride (molar ratio 1: 7.9) were passed over the catalyst in gaseous form at 360 ° C. and 10 bar pressure.

After 6 hours, the conversion was 28.3%, based on the trifluorochloroethane used. The selectivity for tetrafluoroethane was 97.9%, based on the trifluorochloroethane converted.

The reaction product contained 27.7 wt .-% CF₃-CH₂F, 0.006 wt .-% CF₂ = CHCl and 0.6 wt .-% other components in addition to unreacted CF₃-CH₂Cl.

Example 4

The catalyst prepared according to test report A was used in the same test arrangement as in example 1 for the reaction of trifluorochloroethane with hydrogen fluoride. 120 g per hour of trifluorochloroethane and 120 g per hour of hydrogen fluoride were passed over the catalyst in gaseous form at 360 ° C. and 10 bar pressure. Table 1 shows the conversion of trifluorochloroethane and the selectivity for tetrafluoroethane as a function of the reaction time. Table 1 Response time (h) Sales (%) Selectivity (%) 5 27.1 97.2 22 26.5 96.7 32 26.1 97.1 55 26.4 97.5 81 26.6 97.2 90 26.5 97.3

Example 5

The catalyst prepared according to test report B was used in the same test arrangement as in example 1 for the reaction of trifluorochloroethane with hydrogen fluoride. 120 g per hour of trifluorochloroethane and 120 g per hour of hydrogen fluoride were passed over the catalyst in gaseous form at 360.degree.

After 5 hours, the conversion was 29.2%, based on the trifluorochloroethane used. The selectivity for tetrachloroethane was 95.7%, based on the trifluorochloroethane converted.

The reaction product contained 27.9 wt .-% CF₃-CH₂F, 0.28 wt .-% CF₂ = CHCl and 1.02 wt .-% other components besides unreacted CF₃-CH₂Cl.

Example 6

The catalyst prepared according to test report A was used in the same test arrangement as in example 1 for the reaction of trifluorochloroethane with hydrogen fluoride. 120 g per hour of trifluorochloroethane and 120 g per hour of hydrogen fluoride were passed in gaseous form at 420 ° C. over the catalyst.

After 5 hours, the conversion was 36.3%, based on the trifluorochloroethane used. The selectivity for tetrafluoroethane was 81.5%, based on the trifluorochloroethane converted.

The reaction product contained 29.6 wt .-% CF₃-CH₂F, 2.2 wt .-% CF₂ = CHCl and 4.5 wt .-% other components in addition to unreacted CF₃-CH₂Cl.

Claims (6)

1. A process for the preparation of 1,1,1,2-tetrafluoroethane from 1,1,1-trifluoro-2-chloroethane and hydrogen fluoride in the gas phase, characterized in that a catalyst containing chromium and magnesium is used, which is obtainable thereby that one precipitates chromium (III) hydroxide by reacting 1 mol of a water-soluble chromium (III) salt with at least 1.5 mol of magnesium hydroxide or magnesium oxide in the presence of water, converting the reaction mixture into a dough, the chromium hydroxide and a magnesium salt contains, and then dries the dough and treated at temperatures of 20 to 500 ° C with hydrogen fluoride. 2. The method according to claim 1, characterized in that the reaction of 1,1,1-trifluoro-2-chloroethane with hydrogen fluoride in the temperature range of 250-450 ° C is carried out. 3. The method according to claim 1, characterized in that the reaction of 1,1,1-trifluoro-2-chloroethane with hydrogen fluoride in the temperature range of 320-380 ° C is carried out. 4. The method according to any one of claims 1 to 3, characterized in that the reaction of 1,1,1-trifluoro-2-chloroethane with hydrogen fluoride is carried out under a pressure of 1-26 bar. 5. The method according to any one of claims 1 to 3, characterized in that the reaction of 1,1,1-trifluoro-2-chloroethane with hydrogen fluoride is carried out under a pressure of 5-15 bar. 6. The method according to any one of claims 1 to 5, characterized in that hydrogen fluoride and 1,1,1-trifluoro-2-chloroethane are used in a molar ratio of 4: 1 to 8: 1.